1. Field of the Invention
This invention relates to a two-beam optical head. The two-beam optical head of the present invention is particularly suitable for simultaneously performing erasure, recording and reproduction of information by forming two light spots on a recording medium using a monolithic semiconductor laser array, which has two light-emitting points on the same substrate, as a light source.
2. Description of the Related Art
Magnetooptical-disk apparatuses are used as data file apparatuses utilizing the features of a large capacity, and non-contact and high-speed access. When an optical head emitting a single light beam is used in such an apparatus, usually, in order to record data, each of erasure of old information, recording of new information, and reproduction for confirming (verifying) the newly recorded information is performed during a single revolution of a magnetooptical disk. That is, the above-described three operations are completed only after three revolutions of the disk. Alternatively, erasure of old information and recording of new information are simultaneously performed during a single revolution of the disk, and verifying reproduction is performed during the next revolution of the disk. Accordingly, in such a conventional magnetooptical-disk apparatus, a waiting time for the revolutions of a magnetooptical disk is required, which is an obstacle for improving the data transfer speed.
In order to solve the above-described problem, optical heads, which can simultaneously perform erasure, recording and reproduction, or recording and reproduction by emitting a plurality of light beams, have been proposed, for example, in Japanese Patent Laid-open Application (Kokai) No. 58-220247 (1983) (corresponding to U.S. Pat. No. 4,517,887) and No. 64-82348 (1989). FIG. 1 is a schematic diagram of such a conventional optical head emitting a plurality of light beams.
In FIG. 1, monolithic semiconductor laser array 1 has a first light-emitting point 1a and second light-emitting point 1b on the same substrate. Linearly polarized light beams emitted from the respective emitting points are condensed onto magnetooptical disk 6 after passing through substantially the same optical path. That is, the respective light beams emitted from laser array 1 are made to be parallel light beams by collimating lens 2, are condensed by objective lens 4 after passing through beam splitter 3 having a beam shaping portion, and form two light spots 9a and 9b on information-recording surface 7 of magnetooptical disk 6. Light-emitting points 1a and 1b correspond to light spots 9a and 9b, respectively. The two light spots 9a and 9b are disposed so that the light spot 9a leads the light spot 9b on the same information track within information-recording surface 7 of magnetooptical disk 6 rotating in the direction of arrow 11. The light beams reflected by the information-recording surface 7 are condensed again by objective lens 4, are then deflected by beam splitter 3 having the beam shaping portion, and are guided to signal detection unit 8. Reference numeral 10 represents the optical axis of the optical system.
When recording data, light-emitting point 1a emits light so that light spot 9a has a power required for recording, light-emitting point 1b emits light so that light spot 9b has a power sufficient for reproduction, and a magnetic field modulated with data information is applied by an external magnetic head 12. At that time, a so-called information servo-signal for focusing and tracking, and an information-reproducing signal for verifying recording performed by light spot 9a are obtained from light spot 9b. Thus, erasure of old information, recording of new information, and reproduction for verifying the newly recorded information are simultaneously achieved.
In general, in an optical system as shown in FIG. 1, the efficiency of utilization of light of the optical system for advancing light is about 20-50%. The recording power is about 6-10 mW, and the reproducing power is about 1.5 mW. If it is assumed that the efficiency of utilization of light of the optical system for advancing light is 35%, the recording power is 8 mW, and the reproducing power is 1.5 mW, the outgoing power of first emitting point 1a and second emitting point 1b of monolithic semiconductor laser array 1 are 22.9 mW and 4.3 mW, respectively. The power dependency of a semiconductor laser wavelength is 0.1-0.3 nm/mW. If the value of the power dependency of semiconductor laser array 1 is assumed to be 0.2 nm/mW, the wavelength difference .DELTA..lambda. between light beams from first light-emitting point 1a and second light-emitting point 1b of semiconductor laser array 1 is about 3.7 nm. In this case, if each of collimating lens 2 and objective lens 4 is assumed to be a single lens made of molded glass, there is a shift of .DELTA.f between the imaging points by these lenses due to the chromatic aberration of the lenses, as shown in FIG. 2. In FIG. 2, the light beam having a longer wavelength is indicated by broken lines.
Since focus control is performed for reproducing light spot 9b, recording light spot 9a is defocused, whereby the performance of light spot 9a as a recording light spot is deteriorated. On the other hand, if each of collimating lens 2 and objective lens 4 is provided as a combination of a plurality of lenses in order to remove the chromatic aberration, the cost of the optical system increases, the weight of movable components is large, and therefore high-speed access cannot be performed.